8-oxo 2′-deoxyguanosine (hereinafter also called 8-oxo-dG) is attracting attention as an oxidative stress marker because it directly reflects the quantity of reactive oxygen generated by environmental factors or associated with metabolic activities in a living body. Thus, accurate measurement of the quantity of 8-oxo-dG present in a living body or in urine is extremely significant for studies of mutation, aging, and many diseases. Several methods for detecting 8-oxo-dG have been developed, including instrumental analysis such as the HPLC-ECD method, which combines HPLC and an electrochemical detector (see, for example, Non-Patent Document 1) and the GC-MS method, which uses gas chromatography and a mass spectrometer (see, for example, Non-Patent Document 2), and the ELISA method with monoclonal antibodies (see, for example, Non-Patent Document 3).
With these detection methods, however, it is difficult to quantitatively observe the presence of 8-oxo-dG in real time in a living cell.
To overcome the problem, a low-molecular-weight fluorescent probe showing a fluorescence response specific to 8-oxo-dG has been developed (see, for example, Non-Patent Documents 4 and 5).
The low-molecular-weight fluorescent probe recognizes 8-oxo-dG through formation of multiple hydrogen bonds with 8-oxo-dG in an organic solvent and shows specific fluorescence quenching. Thus, the quantity of 8-oxo-dG present in the system can be detected by measuring the degree of fluorescence quenching.
Among a variety of derivatives of the low-molecular-weight fluorescent probe, a low-molecular-weight fluorescent probe showing a fluorescence response specific to 2′-deoxyguanosine (hereinafter also called dG) has been found (see, for example, Non-Patent Document 6).
Shown below are a mode (A) in which the low-molecular-weight fluorescent probe showing a fluorescence response specific to 8-oxo-dG recognizes 8-oxo-dG and forms multiple hydrogen bonds and a mode (B) in which the low-molecular-weight fluorescent probe showing a fluorescence response specific to dG recognizes dG and foams multiple hydrogen bonds.
It is reported that the low-molecular-weight fluorescent probe showing a fluorescence response specific to 8-oxo-dG shown in A has a complexation ability with 8-oxo-dG in a chloroform solution that is 10 times that with dG, and the low-molecular-weight fluorescent probe showing a fluorescence response specific to dG shown in B has a complexation ability with dG in a chloroform solution that is 25 times that with 8-oxo-dG.

The low-molecular-weight fluorescent probe showing a fluorescence response specific to 8-oxo-dG precisely recognizes 8-oxo-dG through multiple hydrogen bonds, and hydrogen bonds hardly act in water that is a protic polar solvent. Thus, the fluorescent probe cannot recognize 8-oxo-dG through multiple hydrogen bonds in water, so it was difficult to directly detect highly water-soluble 8-oxo-dG in an aqueous solution such as urine.
In addition, the usual concentration of 8-oxo-dG in urine is reported to be extremely low of 10 ng/mL to 20 ng/mL (corresponding to 35 nM to 70 nM assuming a molecular weight to be 283). Therefore, the low-molecular-weight fluorescent probe needs to be improved for higher sensitivity to enable detection of 8-oxo-dG at lower concentrations, in order to use the probe for detection of 8-oxo-dG in urine.